Materials that are needed by humans are present as solids, liquids and gases in the sea, land and air, and are also present as independent molecules or compounds. Desired materials can be obtained by catalytic reactions, chemical reactions, biological reactions, etc.
For example, salt that is most frequently used by humans is present in sea water at a concentration of 3.0 wt %. 1000 g of sea water consists of 30 g of salt and 970 g of water. If 500 g of water is removed therefrom, 30 g of salt and 470 g of water remain, and if 250 g of water is additionally removed, 220 g of water and 30 g of salt remain. If 125 g of water is further removed, 95 g of water and 30 g of salt (28%) remain. If 62.5 g is subsequently removed, 22.5 g of water and 30 g of salt remain, and in this case, the content of salt is 57.1 wt %. However, because the maximum solubility of salt is 26.4% (359 g/1000 g water), water is continually removed, but salt starts to precipitate as a crystal before 22.5 g of water and 30 g of salt are reached.
In the case of organic acids, when easily decomposable animal and vegetable materials such as food waste start to be anaerobically decomposed, polymers are converted into single molecules which then form mixtures of organic acids (acetic acid, propionic acid, butyric acid, etc.), resulting in methane gas and carbonic acid gas. Herein, the concentration of organic acids that can be obtained by fermentation is known to about 35 g/L (45 g/L, sodium salt). However, in order to use these organic acids as a raw material for biofuel (bioalcohol, biodiesel, etc.), an additional process for concentrating the organic acids is required. US Patent Publication No. 2012-0118827 discloses concentrating an organic acid about four times (140 g/L of solvent) using forward osmosis.
Methods of further concentrating a concentrated organic acid include a method in which a mixture of calcium and organic acid is prepared using calcium hydroxide (Ca(OH)2), followed by heating. When this method is used, the organic can be concentrated to 100%, and when 98% sulfuric acid (H2SO4) is added thereto, calcium as calcium sulfate (CaSO4) can be precipitated as a salt. Organic acids can be obtained by fermentation of organic matter, and particularly, an organic acid mixture comprising acetic acid, propionic acid and butyric acid at a ratio of 8:1:1 6:1:3 or 5:1:5 depending on a fermentation process and a raw material can be produced.
Bioethanol is produced as a solution containing 6-10% ethanol from maize starch, sugar canes, etc.
Metha (1982) concentrated a 7.6% ethanol-containing solution to 20-30% by reverse osmosis at 60 atm, concentrated the concentrated solution to 95% by distillation, concentrated the distilled solution to 99.5% by azeotropic distillation to obtain ethanol for fuel, and compared the economic efficiency of this method with that of a method that uses distillation from beginning (see Metha, G D, Journal of Membrane Science, 12, 1-26 (1982).
Energy required for the concentration of salt, volatile fatty acids (VFA) and ethanol in the above-described examples is shown in Table 1 below.
TABLE 1Reason forMaterialThermal methodUse of membranelimitSaltPossible (up toUp to 7%Osmotic100%)pressure offeed solutionVFAPossible (up toUp to 14%Osmotic100%)(forward osmosis)pressure offeed solutionEthanolPossible (up toPossible only atOsmotic95%)low concentrationpressure of(20-30%)feed solution
Energy required to remove 1000 g of water by a thermal method is 730 kwh, because energy required to evaporate 1 m3 of water at 30° C. to steam 100° C. is 2.7×109 joule and 1 kwh is 3.6×106 joule. Thermal energy can be used several times, and thus the multi-stage flash (MSF) process requires energy of about 25 kwh (http://en.wikipedia.org/wiki/Multi-Stage_Flash) accessed on Nov. 10, 2012).
Meanwhile, membrane processes (reverse osmosis and forward osmosis) requires energy of 2.5 mJ to remove 1 m3 of water, and this amount of energy corresponds to 0.69 kwh (=2.5 mJ/3.6 mJ). The multi-stage flash (MSF) process having the highest efficiency among thermal processes requires energy of 25 kwh, whereas the use of membranes requires energy of 0.69 kwh, indicating that the application of the membrane process leads to an increase in economic efficiency (http://en.wikipedia.org/wiki/Multi-stage_flash_distillation).
Currently, the membrane process is applied not only to reverse osmosis, but also to forward osmosis, and thus is used in many industrial fields. In the membrane process, the flux of a solvent (water) and the movement of a solute (salt, VFA, ethanol, etc.) are as follows.Jw=Lp(ΔP−σΔπ)  (1)Js=Cs(1−σ)Jw+ωΔπ  (2)wherein Jw is water flux; Lp is water permeability coefficient; ΔP is the hydraulic pressure difference between a feed chamber and a draw chamber; Δπ is the osmotic pressure difference between the feed chamber and the draw chamber; and Js is the flux of the solute, which is divided into one caused by Jw and one caused by the osmotic pressure difference.
If Jw is not present in equation (2) above, the solute can move from the draw chamber to the feed chamber due to the osmotic pressure difference. σ is the reflection coefficient of the solute by the membrane, and at σ=1, the solute is completely impermeable, and the osmotic pressure difference between the two chambers reaches the maximum.
Osmotic pressure is expressed as the following equation (3):π=CRT  (3)wherein C is concentration; R is gas constant; and T is temperature.
In addition, the Lewis equation for a solution containing a high concentration of a solute is expressed as the following equation 4:π=RT/vsp ln(1−γX)  (4)wherein Vsp is the volume of 1 mole of a solvent when the concentration of a solute is 0; γ is the activity coefficient of the solvent; and X is the molar fraction of the solute (References: Lewis, G. N., The osmotic pressure of concentrated solutions and the laws of perfect solution. Journal of the American Chemical Society 1908, 30, 668-683.).
If 30 g/L of a solute is dissolved in water, the osmotic pressure of the solute is 25.4 bar for salt, 0.01 bar for albumin, and 1.2×10−12 bar for particles.
The reverse osmosis and reverse osmosis processes have an advantage in that energy is saved due to the use of membrane, but have a disadvantage in that, as concentration progresses, the osmotic pressure in the feed chamber increases so that it is impossible to further concentrate the feed solution or to increase the utility of the feed solution (Loeb, S, Loeb-Sourirajan Membrane, How it Came About Synthetic Membranes, ACS Symposium Series, 153, 1, 1˜9, 1981; Loeb, S., J. Membr. Sci, 1, 49, 1976).
The forward osmosis process that recently started to be studied is a process in which a material having high osmotic pressure is used in the draw chamber so that only water will move from the feed chamber to the draw chamber due to Δπ rather than ΔP (McCutcheon J R, McGuinnis R L, Elimelech R L, Desalination, 174, 1˜11, 2005).
The forward osmosis process has an advantage in that the osmotic pressure of the draw chamber is maximized. However, as the amount of water moving to the draw chamber increases, Δπ decreases gradually, and thus the amount of permeate decreases gradually. In this case, the draw solution can be regenerated, but this regeneration is not economic. In addition, there is a disadvantage in that, as the concentration of the draw solution in the draw chamber increases, the solute in the draw solution is diffused back into the feed chamber through the membrane.
Accordingly, the present inventors have made extensive efforts to solve the above-described problems, and as a result, have found that, when a solute-containing aqueous solution is introduced into the feed chamber of a concentrator comprising the feed chamber and a draw chamber, which are separated from each other by a reversal osmosis membrane and/or a forward osmosis membrane, and when a solution having the same osmotic pressure as that of the aqueous solution introduced into the feed chamber is introduced into the draw chamber, Δπ in equations (1) and (2) above can be eliminated or minimized so that the feed solution can be concentrated even by hydraulic pressure alone under a zero osmotic pressure difference condition (Δπ=0) or a low osmotic pressure difference condition, and for this reason, the diluted draw solution will have low osmotic pressure so that pure water can be recovered by reverse osmosis, and ultimately, energy consumption and management costs can be minimized while concentration of the feed solution can be maximized, thereby completing the present invention.